drm: Mark up racy check of drm_gem_object.handle_count

[ 1715.899800] BUG: KCSAN: data-race in drm_gem_handle_create_tail / drm_gem_object_handle_put_unlocked
[ 1715.899838]
[ 1715.899861] write to 0xffff8881830f3604 of 4 bytes by task 7834 on cpu 1:
[ 1715.899896]  drm_gem_handle_create_tail+0x62/0x250
[ 1715.899927]  drm_gem_open_ioctl+0xc1/0x160
[ 1715.899956]  drm_ioctl_kernel+0xe4/0x120
[ 1715.899981]  drm_ioctl+0x297/0x4c7
[ 1715.900003]  ksys_ioctl+0x89/0xb0
[ 1715.900027]  __x64_sys_ioctl+0x42/0x60
[ 1715.900052]  do_syscall_64+0x6e/0x2c0
[ 1715.900079]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 1715.900100]
[ 1715.900119] read to 0xffff8881830f3604 of 4 bytes by task 8137 on cpu 0:
[ 1715.900149]  drm_gem_object_handle_put_unlocked+0x31/0x130
[ 1715.900180]  drm_gem_object_release_handle+0x93/0xe0
[ 1715.900208]  drm_gem_handle_delete+0x7b/0xe0
[ 1715.900235]  drm_gem_close_ioctl+0x61/0x80
[ 1715.900264]  drm_ioctl_kernel+0xe4/0x120
[ 1715.900291]  drm_ioctl+0x297/0x4c7
[ 1715.900316]  ksys_ioctl+0x89/0xb0
[ 1715.900340]  __x64_sys_ioctl+0x42/0x60
[ 1715.900363]  do_syscall_64+0x6e/0x2c0
[ 1715.900388]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200309120151.7675-1-chris@chris-wilson.co.uk